U.S. patent application number 12/934721 was filed with the patent office on 2012-01-26 for method for inertial navigation under water.
Invention is credited to Guenter Schmitz, Tim Schmitz.
Application Number | 20120022820 12/934721 |
Document ID | / |
Family ID | 40933762 |
Filed Date | 2012-01-26 |
United States Patent
Application |
20120022820 |
Kind Code |
A1 |
Schmitz; Guenter ; et
al. |
January 26, 2012 |
METHOD FOR INERTIAL NAVIGATION UNDER WATER
Abstract
The invention relates to a method for underwater navigation,
particularly for scuba divers, and for autonomous, manned, or
remote-controlled underwater vessels, wherein the signals of at
least one sensor, comprising at least one acceleration sensor for
determining the actual position, are integratively analyzed,
accuracy is improved by means of the use of reference measurements,
and a correction is carried out by way of a correction vector
obtained from the transformation of the vector of the accelerations
measured by the acceleration sensor in the diving computer
coordinate system into the global coordinate system, the comparison
to at least one of the reference measurement values, the
determination of the deviation and the reverse transformation of
the deviation to the diving computer coordinate system.
Inventors: |
Schmitz; Guenter; (Aachen,
DE) ; Schmitz; Tim; (Aachen, DE) |
Family ID: |
40933762 |
Appl. No.: |
12/934721 |
Filed: |
April 17, 2009 |
PCT Filed: |
April 17, 2009 |
PCT NO: |
PCT/EP2009/002843 |
371 Date: |
October 17, 2010 |
Current U.S.
Class: |
702/95 |
Current CPC
Class: |
G01C 21/165
20130101 |
Class at
Publication: |
702/95 |
International
Class: |
G06F 15/00 20060101
G06F015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 17, 2008 |
DE |
10 2008 019 444.1 |
Claims
1. A method of underwater navigation of scuba divers or autonomous,
manned or remotely controlled underwater vehicles wherein the
signals of at least one sensor, comprising at least one
acceleration sensor for determining the actual position are
integratively analyzed, the precision is improved by utilizing
reference measurements, and a correction takes place by a
correction vector that is obtained from the transformation of the
vector of accelerations measured by the acceleration sensor in the
diving computer coordinate system in the global coordinate system,
comparison with at least one of the reference measurement values,
determination of the deviation and the reverse transformation of
the deviation into the diving computer coordinate system.
2. The method according to claim 1, wherein the signals of a
combination resulting from the at least one, in particular
straight-line acceleration sensor and at least one angle of
rotation or line-of-site rate sensor or angular acceleration sensor
are integrated.
3. The method according to claim 1 wherein the reference
measurement takes place in the form of a pressure measurement of
the ambient water pressure.
4. The method according to claim 1 wherein the reference
measurement is performed in the form of utilizing signals of a
satellite navigation system.
5. The method according to claim 4, wherein the signals of the
satellite navigation system are utilized as reference when at or in
the proximity of the water surface, in particular directly prior or
after starting the dive.
6. The method according to claim 1 wherein the reference
measurement is performed by utilizing signals from the measurement
of the earth's magnetic field.
7. The method according to claim 1 wherein a navigation aid is
displayed for the diver on a display.
8. The method according to claim 7, wherein the display provides
information about the direction or distance to a reference
point.
9. The method according to claim 8, wherein the reference point is
the starting point of the dive.
10. The method according to claim 8, wherein the display has an
illustration that is similar to a map.
11. The method according to claim 7, wherein the display
additionally shows a map that was previously loaded into the device
or previously recorded or loaded or recorded reference points.
12. The method of recording the path traveled under water according
to claim 1 wherein position information is recorded from
acceleration sensors or angular sensors or line-of-sight rate
sensors or magnetic field sensors or pressure sensors for storing
position information depending on the time or a meter reading.
13. The method according to claim 1, wherein reference positions
are stored in the memory of the diving computer prior to or during
the dive.
14. The method according to claim 1, wherein the position of a
diver is forwarded to another diver, in particular by radio
communication or other types of communication.
15. The method according to claim 1, wherein pressure depth is
corrected by information about the salt content of the water.
16. The method according to claim 15, wherein pressure depth and
depth from other calculations are compared and that the salt
content is determined from such.
17. The method according to claim 1, wherein a calibration of the
map direction takes place in a map in the diving computer, in
particular a stored map by a a determination of the direction north
by a magnetic compass, analysis of the movements of the diver when
a GPS signal is available, or diving to at least one reference
point and confirmation by the diver to the diving computer that
this at least one point was reached.
18. The method according to claim 1, wherein the calibration of the
absolute position is derived from an at least one momentarily
received GPS signal.
19. The method according to claim 1, wherein a correction of the
delay time of the GPS data with respect to the propagation
properties of the GPS signals takes place under water.
20. The method according to claim 1, wherein a measured temperature
is used for error correction.
21. The method according to claim 1, wherein an average value
formation of the depth occurs in order to reduce the influence of
the waves on the depth determination.
22. The method according to claim 1, wherein a correction of the
measured values obtained by the angular sensors is performed by the
direction of the gravitational force.
23. The method according to claim 1, wherein a recalibration of the
system takes place in calm phases.
24. The method according to claim 1, wherein the system is divided
into measuring unit and display unit and that the measuring unit is
attached to a part of the diver or his equipment, in particular a
part that is not moved very much.
25. The method according to claim 1, wherein gravity is corrected
by the information captured about the degree of latitude.
26. The method according to claim 1, wherein the temperature or
salt content of the water or the date or the elevation are included
in the determination of the degree of latitude or the gravitational
constant prior to the dive.
27. The method according to claim 1, wherein a determination of the
degree of latitude takes place in addition to or exclusively by the
analysis of the earth's magnetic field.
28. The method according to claim 1, wherein from the amplitude of
the pressure fluctuations a degree of waviness is derived.
29. The method according to claim 28, wherein the degree of
waviness that was determined is stored.
30. The method according to claim 1, wherein the position of the
diver is transmitted to a remote receiver.
31. The method according to claim 1, wherein the receiver is
another diver or is on a boat or is on land.
32. The method according to claim 1, wherein the controls of an
underwater propulsion device are controlled based on position data.
Description
DESCRIPTION OF THE INVENTION
[0001] The invention relates to a method of inertial navigation
under water, in particular for scuba divers.
BACKGROUND OF THE INVENTION
[0002] Because of the limited range of visibility under water,
which is at a maximum of 30-40 m most of the time and often
significantly lower (2-10 m), orientation under water is often very
difficult. During a customary diving time of between 30 and 60
minutes, the diver moves up to 500 m away from his entry point (for
example a boat). In spite of bad visibility, he must be able to
find the boat again in order to not end up in a life-threatening
situation. Up to now, divers have had to depend on the use of
simple compasses, that only indicate the direction of north,
without registering the distance traveled.
[0003] Because of this difficulty, systems for determining boat
position by ultrasound have been developed (so-called boat
finders). Two-part systems of this type consisting of transmitter
and receiver are cumbersome to handle, as the transmitter must be
fastened to the boat at the start of the dive.
[0004] DE 3742423 describes a boat finder using ultrasound. U.S.
Pat. No. 3,944,977, U.S. Pat. No. 3,986,161 and U.S. Pat. No.
5,570,323 also describe systems of this type. A further
disadvantage of systems of this type is inherent therein, that a so
to speak direct "sight connection" must exist with the boat, as the
ultrasound signal cannot be go through rock spurs or other
obstacles, or even indicates an incorrect direction.
[0005] A system would be much more useful that knows the position
of the diver at least relative to the entry point and can thus
display information about the direction and distance relative to
the entry point. To do so, the path traveled by the diver would
have to be recorded.
[0006] However, navigation systems based on GPS are not suitable
because of the low depth of penetration of the satellite signals
under the water surface (see also EP 1631830 [U.S. Pat. No.
6,972,715], U.S. Pat. No. 6,701,252, U.S. Pat. No. 6,791,490 and
U.S. Pat. No. 6,807,127).
[0007] Inertial navigation systems are known from aviation and
space flight, but they are out of the question for this
application, because of their size and costs.
[0008] Innovative, micromechanical acceleration sensors are,
however, in a position to measure precise straight-line and
rotatory accelerations or angular velocity. By integrating these
signals, the three-dimensional path that was traveled can be found
and from it, the direction and distance from the initial position
can be determined.
[0009] In EP 0870172 [U.S. Pat. No. 6,308,134], a vehicle
navigation system is described using acceleration sensors in which
a GPS signal is used for calibration.
[0010] As, however, no GPS system is available for calibration
under water, the calibration must be performed by a different
signal.
[0011] A small offset or sensitivity error can lead to an error of
easily several hundred meters after only 10 minutes of diving time
by using an inertial navigation system, because of the required
double integration of the measured acceleration signals.
[0012] These types of sensor errors can be electronically
compensated for by reference signals such as the measured ambient
pressure (depth information) and if necessary, by a magnetic
compass and perhaps additionally by the ability to receive GPS
signals (for example, at or near the surface).
[0013] In US2007/0006472 A1, such a system is described. However,
the method of correction remains unpublished. It is merely stated
that additionally measured values are fed into the system in order
to calculate back to the inertial error vector. How the inertial
error vector is found is not disclosed.
SUMMARY OF THE INVENTION
[0014] The present invention is a method of underwater navigation
for scuba divers, as well as for autonomous, manned or remotely
controlled underwater vehicles in which the signals of one or
several, in particular straight-line acceleration sensors, as well
as angle of rotation sensors and/or angular acceleration sensors
and/or line-of-sight rate sensors for determining the actual
position are integratively analyzed and hence precision is improved
by utilizing reference measurements, by making a correction by a
correction vector that is obtained from the transformation of the
vector from an acceleration sensor, in particular a straight-line
acceleration sensor--accelerations measured in the diving computer
coordinate system in the global coordinate system, comparison with
at least one of the reference measurement values, determination of
the deviation and the reverse transformation of the deviation into
the diving computer coordinate system.
[0015] The defective acceleration vectors of at least one of the
acceleration sensors (for example, a straight-line acceleration
sensor or also an angular acceleration sensor) can, for example, be
corrected thereby, that a correction vector is found that is
applied to the defective acceleration vector, whereby the
correction vector can be found as follows:
[0016] A transformation of the defective acceleration vector takes
place in the global coordinate system, a double integration of at
least one selected defective coordinate of the transformed
acceleration vector, the formation of the error magnitude of this
selected coordinate at least by a specifically measured error-free
reference value, in particular by subtracting at least the
reference value from this selected coordinate, a determination of
the error magnitudes of the remaining coordinates by the corrected
coordinates determined in a previous step (in particular by
calculating the difference) a double differentiation of the error
magnitude of the selected coordinate and reverse transformation of
the error magnitudes of all coordinates into the diving computer
coordinate system, whereby the correction vector is formed from the
reverse-transformed error magnitudes of the diving computer
coordinate systems.
DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 shows a system for determining and recording the
position information with the help of an inertial navigation system
INS;
[0018] FIG. 2 shows the principle of the implementation of the
correction method in accordance with the invention;
[0019] FIG. 3 shows the more precise progression of the
correction;
[0020] FIG. 4 shows an example of a possibility for finding a
confidence factor;
[0021] FIG. 5 shows an example for a weighting function for
suppressing correction values that are not sufficiently
reliable;
[0022] FIG. 7 shows an alternative embodiment to that shown in FIG.
3.
[0023] FIG. 8 shows an additional alternative.
DETAILED DESCRIPTION OF THE EMBODIMENT
[0024] The following will present a more precise description of the
method in accordance with the invention. First, however, the terms
"diving computer coordinate system" and "global coordinate system"
will be addressed briefly.
[0025] The diver takes along the diving computer most often either
on the arm or in a console. In the process, alignment of the
computer with respect to the environment (global position)
continually changes. Thus, for the determination of the movement in
the "environment coordinate system" or "global coordinate system,"
i.e. the motion or the acceleration forces that act upon the diving
computer must be converted from the diving computer coordinate
system into the global coordinate system.
[0026] The diving computer coordinate system can, in principle, be
determined arbitrarily, for example, dependent on the insertion
position (or insertion orientation) of the measuring chip used as
sensor. For the sake of simplicity it can, for example, be assumed
that the coordinate system of the diving computer is oriented in
such a way that when the viewer looks at the display of a
horizontally positioned diving computer precisely from above, the x
axis points to the right, the y axis points "up" relative to the
eyes of the viewer, and the z axis points precisely into the eye.
These axes will be described in the following description as
x.sub.T, y.sub.T and z.sub.T.
[0027] In principle, the global coordinate system can also be
selected completely arbitrarily. Here, it represents, relative to
any map projection where the x axis points east, the y axis north
and the z axis points perpendicular up out of the earth's surface
and z=0 represents the actual water surface, for example, sea
level. The axes of the global coordinate system will be described
in the following by x.sub.w, y.sub.w, and z.sub.w.
[0028] FIG. 1 shows a system for determining and recording the
positioning information with the help of an inertial navigation
system INS.
[0029] A triaxial acceleration sensor 1 (S), as well as a triaxial
angular acceleration sensor 2 (RS=rotation sensor) make environment
information available. These data are converted in the path
calculation unit 12a into a path and are fed to the recording
device 5. In detail, the analysis is performed using the following
steps:
[0030] The raw data of the angular acceleration sensor 2 and the
three angular accelerations j, z, and y are converted by double
integration in integrator block 2a into solid angles and in the
angle correction block 6, converted into the final solid angles
.phi., .THETA., .PSI.. How this correction is performed will be
described later.
[0031] The raw data of the triaxial acceleration sensor 1 are
naturally present as acceleration values in the coordinate system
of the diving computer. These acceleration data are labeled
x.sub.T, y.sub.T and z.sub.T, where T identifies the diving
computer coordinate system. The lower case letters indicate that
these are accelerations. Upper case letters are used here to
distinguish position information.
[0032] With the help of a transformation matrix 3 (T), the
acceleration values from the diving computer coordinate system are
transformed into the global coordinate system. The acceleration
values of the global coordinate system x.sub.w, y.sub.w and z.sub.w
are thus obtained. The Z axis of the global coordinate system
points in the direction of the geocenter, i.e. "down." For this
reason, for the analysis of the movement in the Z direction, first
the gravitational acceleration 10 must be subtracted. It is
approximately 9.81 m/s.sup.2.
[0033] Subsequently, the acceleration in one path can be converted
into a path with the help of a double integration in integrator
block 1a (coordinates X.sub.w, Y.sub.w, Z.sub.w). This is conveyed
to a recording device 5 (log) for the (three-dimensional) path.
There, the path information is then available for recording and
additional utilization to provide information about the return
path, etc.
[0034] In total, the matrix 3 (T) performs the following
operation:
( x w y w z w ) = T ( x T y T z T ) ##EQU00001##
[0035] The transformation matrix 3 (T) can consist of individual
matrices for the individual rotations. Then, somewhat more clearly
arranged relationships result, which are easier to understand.
[0036] Matrix T can be formed using the individual transformations
around the respective axes:
T=T.sub.xT.sub.yT.sub.x
where:
T x = ( 1 0 0 0 cos .PHI. - sin .PHI. 0 sin .PHI. cos .PHI. ) , T y
= ( cos .THETA. 0 sin .THETA. 0 1 0 - sin .THETA. 0 cos .THETA. ) ,
T z = ( cos .PSI. - sin .PSI. 0 sin .PSI. cos .PSI. 0 0 0 1 )
##EQU00002##
whereby .phi. represents the angle of rotation around the x axis,
.THETA. the angle of rotation around the y axis, and .PSI. the axis
of rotation around the x [z] axis.
[0037] The method according to FIG. 1 is known. FIG. 2 shows the
principle of the implementation of the correction method in
accordance with the invention. This figure was inserted to improve
the understanding of following FIG. 3 in which the progression of
the correction is shown in greater detail. For reasons of
simplification, the determination of the solid angles with respect
to FIG. 1 was not shown. Only block 6 is shown for inputting the
angles into the transformation matrix 3 (T). With respect to the
path calculation unit 12a from FIG. 1, the path calculation unit
12b is expanded as follows: A correction block 11 is connected
between the acceleration sensor 1 and the transformation matrix 3,
the error of the acceleration values of the sensor unit 1 with
respect to offset and linearity is corrected.
[0038] For the three axes, these errors are labeled .DELTA.x,
.DELTA.y and .DELTA.z. The sensor unit 1 thus supplies the
defective signals x.sub.T.times..DELTA.x, y.sub.T+.DELTA.y and
z.sub.T+.DELTA.z. In the correction block 11, the errors of these
signals are removed. Even if the illustration suggests here that
only offset errors are removed and none of the linearity errors,
these are also corrected, as in the analysis unit 9 (FIG. 3), both
error types are determined and are being considered in the
correction unit. However, for the sake of clarity, the illustration
in FIG. 2 was kept as simple as possible.
[0039] FIG. 3 shows the determination of the correction values. The
path calculation unit 12 of FIG. 3 corresponds closely to the path
calculation unit 12b of FIG. 2. However, here, the Z coordinate is
not determined from the acceleration sensor signals, but is taken
directly from depth information 7, that comes from the pressure
analysis of the diving computer for determining the diving
depth.
[0040] This "pressure depth," which is to be viewed as being
correct is now also used in order to find the errors of the sensor
signals in a correction value calculation block 13.
[0041] The function of the correction value calculation block 13 is
as follows:
[0042] The defective sensor data x.sub.T+.DELTA.x, y.sub.T+.DELTA.y
and z.sub.T+.DELTA.z are first converted from the diving computer
coordinate system into the global coordinate system using
transformation matrix 3a. This transformation matrix is identical
to transformation matrix 3. Now, the defective acceleration values
x.sub.W+.DELTA.x', y.sub.W+.DELTA.y' and z.sub.W+.DELTA.z' are
available in the global coordinate system as output of
transformation matrix 3a. The identification of the error variables
.DELTA.x', .DELTA.y', .DELTA.z' with an apostrophe is to make it
clear that these are not the original error values .DELTA.x,
.DELTA.y and .DELTA.z.
[0043] Special attention only needs to be paid to the value of the
Z direction, i.e. the "depth direction." This value is first
corrected, again by the value of the gravitational acceleration 10,
by subtraction. Next, a calculation of the (defective) depth takes
place in integrator 1b by double integration of the acceleration
value. Now, a difference with respect to the depth 7 that was found
by the pressure measurement is determined. The thus obtained error
value for the depth is converted into an error value for the
acceleration in the z direction .DELTA.z', by differentiating two
times in differentiator 8.
[0044] The two other channels for the x and the y acceleration are
reduced with the help of the actually determined corrected values
for x and y in the global coordinate system reduced to their
absolute error magnitude. For this, the magnitude x.sub.w is
subtracted from x.sub.w.DELTA.x' and the magnitude y.sub.w is
subtracted from y.sub.w+.DELTA.y', and .DELTA.x' and .DELTA.y'
remains. These are, together with the error value .DELTA.z', fed to
a transformation matrix 3a (T) that is inverse to transformation
matrix 4 (T.sup.-1). The error magnitudes .DELTA.x'', .DELTA.y''
and .DELTA.z'' now result as its output, which are now present in
the coordinate system of the diving computer as a result of the
reverse transformation. These values are fed to an analysis unit 9
for the determination of the correction factors.
[0045] This analysis unit 9 must now determine the correction
values .DELTA.x.sub.K, .DELTA.y.sub.K and .DELTA.z.sub.K.
[0046] This analysis is shown for the x component in FIGS. 4 to 6;
it applies equally, however, to all other components. When
determining the correction values it is important that the
respective relevance of the magnitude of the correction is
determined, because only the part coming from the depth information
provides a real possibility for correction. Depending on the angle
between the diving computer coordinate system and the global
coordinate system, the depth information originates, however, in
different sensors.
[0047] It is preferably provided that only one sensor, which is
involved to a high degree in making depth information available, is
corrected. For this reason, a "relevance" or "confidence" factor c,
is introduced that is determined for each individual sensor from
the actual solid angle.
[0048] A corresponding weighting vector for the depth information
can, for example, be determined from the transformation of a vector
that has only one component in the Z direction.
[0049] For example, FIG. 4 shows such a possibility for determining
a confidence factor. A vector that is only assigned to `1` in the Z
direction is applied to the input of an inverse transformation
matrix 4a, and thus transformed by the global coordinate system
into that of the diving computer. At the output of the inverse
transformation matrix 4a, there now result factors that indicate to
which degree the respective sensor (of the diving coordinate
system) had participated in the formation of the depth value (Z
axis global) (participation value c.sub.B)
[0050] If one sensor did not participate at all, the value of
c.sub.B is `0`. If only one sensor was involved, this value is `1`,
or also minus `1` (at 180.degree. rotation of the angle). In the
case of intermediate angles, corresponding intermediate values
result. In an optional further development it can be provided that
behind the inverse transformation matrix 4a, still and additional
"weight factor" 14 is connected or applied to the obtained values
c.sub.B that forms a non-linear connection between output and
input. Thus, it can be provided, in the range of low participation
values c.sub.B, in is particular in the range of participation
values below the respectively specified or predeterminable limit
value that these confidence factors c.sub.C are set to zero, in
order to minimize the influence of other error values.
[0051] One example of a weighting function of this type is shown in
FIG. 5. The corresponding calculation rule is:
[0052] If |c.sub.B|.ltoreq.0.5 then: c.sub.C=0, thus, the value 0.5
is the cited limit value here
[0053] If |c.sub.B|>0.5 then: c.sub.C=2(|c.sub.B|-0.5)
[0054] It can thus be provided that for participation values
c.sub.B that are specified above a predetermined or predeterminable
limit value, the respective confidence factor is specified by a
calculation rule, in particular dependent on the participation
value. This proposed weighting function is only an example that has
provided good results in practice. Alternative functions such as,
for example, a quadratic function are, however, equally possible
and included in the scope of the patent claims.
[0055] Thus, even a very simple function that is below a certain
value for c.sub.B `0` and above `1` can be used. This then
corresponds to a decision to let the correction become only
effective then, when the direction of the corresponding
acceleration sensor sufficiently agrees with the corresponding
direction in which the direction of capture of the correction
device (i.e. most often the depth, is in the z direction).
[0056] To determine the correction values .DELTA.x.sub.K,
.DELTA.y.sub.K, and .DELTA.z.sub.K, an algorithm for calculating
the found confidence factors c.sub.CX, c.sub.CY, c.sub.CZ, and the
error magnitudes .DELTA.x'', .DELTA.y'' and .DELTA.z'', must still
find application. Thereby, it is seen to be preferable, when the
period of time during which an error is present is also included in
the analysis.
[0057] In an advantageous embodiment, a digitally sliding average
value formation is used, in which the algorithm follows the
principle of calculating a new average value by including the new
initial value only at a certain percentage P, and the old average
value at the remaining percentage 100%-P.
[0058] If the current initial value is labeled a," the old average
value as a(k-1) and the new one as a(k), the following calculation
rule results:
a(k)=a''P+a(k-1)(100%-P)
[0059] The chronological progression of such an average value
formation depends on the one hand on the frequency of the execution
(scanning rate) of this operation, and on the other hand, on the
size of factor P. In the case of a high scanning rate and at a high
percentage, a very fast adaptation of the average value to the new
initial value results. In this connection, an accommodation can be
made by introducing an "average value constant" c.sub.M, that
results from the scanning rate or the scanning interval T.sub.A and
the percentage P:
c.sub.M=PT
[0060] The confidence factor should--as described previously
already--be included when building the average values of the
correction values. This is achieved very easily by also including
this factor when building the average value, in the same manner as
the average value factor. The following is obtained:
a(k)=a''c.sub.Mc.sub.C+a(k-1)(1-c.sub.Mc.sub.C)
[0061] When applying this algorithm directly to the error
magnitudes .DELTA.x'', .DELTA.y'' and .DELTA.z'', the following
correction values .DELTA.x.sub.K, .DELTA.y.sub.K and .DELTA.z.sub.K
result as follows:
.DELTA.x.sub.K(k)=.DELTA.x''c.sub.Mc.sub.Cx+.DELTA.x.sub.K(k-1)(1-c.sub.-
Mc.sub.Cx)
.DELTA.y.sub.K(k)=.DELTA.y''c.sub.Mc.sub.Cy+.DELTA.y.sub.K(k-1)(1-c.sub.-
Mc.sub.Cy)
.DELTA.z.sub.K(k)=.DELTA.z''c.sub.MMc.sub.Cz+.DELTA.z.sub.K(k-1)(1-C.sub-
.Mc.sub.Cz)
[0062] FIG. 6 [5] shows the application of the analysis method
illustrated here on the x axis in a graph. In the analysis unit 9a
for which the x axis (in the diving computer coordinate system),
the error magnitude .DELTA.x'' (18) first gets to a multiplier 15a
by being multiplied with the product of the average value constant
c.sub.M (21) and the confidence factor c.sub.Cx (16) (formed in
multiplier 15b). The second component for newly formed correction
value .DELTA.x.sub.K (18) in summarizing unit 19, results from the
product formed in multiplier 15c by the delayed initial value
18--delayed by delay element 20--and the product of c.sub.M and
c.sub.C that is subtracted from 1.
[0063] The correction factor in turn, can be broken down further in
an iterative process into an offset part and a product part. Based
on the fact that the offset acts primarily in the range of smaller
acceleration values and the product part acts primarily in the
range of large acceleration values, the defective value for
x.sub.T, here labeled as x.sub.T', can be expressed as follows:
x.sub.T'=.DELTA.o.sub.x+(1+.mu.m.sub.x)x.sub.T
where .DELTA.o.sub.x represents the offset and .DELTA.m.sub.x the
product part (ascending part).
[0064] The iterative calculation method of the offset and the
product correction values can be performed with the following
equations:
.DELTA.m.sub.x=(.DELTA.o.sub.x-.DELTA.x.sub.K)/x.sub.T
.DELTA.o.sub.x=.DELTA.x.sub.K-.DELTA.m.sub.xx.sub.T
[0065] Processing of other reference signals
[0066] In the event other reference signals than the depth
information are available, these can be processed in a similar
manner. The confidence factor is then separately calculated for
each individual component that is to be corrected. For this,
respectively the initial vector for the inverse transformation
matrix in FIG. 4a is calculated with `1`, not in the z direction,
but in the respectively relevant spatial direction assigned to `1`,
and in the spatial directions that are not relevant assigned to
`0`.
[0067] Limiting and alerting in the case of initial sensor values
that are too high
[0068] Should the measured acceleration values (or in the case of
angle sensors the correspondingly measured angular velocities) be
above a predeterminable threshold value, an erroneous measurement
must be assumed. In this case, the diver should be made aware in
the display of the diving computer or in another suitable way, so
that he knows that the determination of additional positions is
perhaps erroneous. Further, the diver can also be asked (for
example by an acoustic alarm signal), to remain at rest for a short
period of time, so that the sensor can reset the integrators, and
so that no erroneous velocity information can lead to errors in the
further position calculation.
[0069] Angle Correction
[0070] In principle, the angle correction takes place in the same
way as previously described for the straight-line accelerations.
Thereby, a magnetic sensor (electronic compass) based on the
earth's magnetic field can be used as reference signal. A further
possibility consists of the utilization of the direction of the
gravitational vector that always points in the direction of the
axis of the earth with a deviation from the plumb line of only at a
maximum 0.01.degree.. For this, the average directional vector of
the maximum acceleration can be used. This can in particular also
take place when the diving computer is at rest, i.e. is not being
moved, which can be derived from the unchanging signals for
straight-line or rotatory accelerations. Perhaps a precise
recalibration of the angle sensors can take place from time to
time, by switching off the diving computer at first delayed, or
even by switching it on automatically from time to time, or
awakening it automatically from a resting position. Even today's
diving computers continue to operate in resting position in order
to perform monitoring of the ambient pressure or a calculation of
the so-called desaturation times. Even the recalibration of the
straight-line sensor can be performed in this mode.
[0071] Deviating from the application of the angle acceleration
sensors shown in FIG. 1 and the following, special advantages can
be achieved by using line-of-sight rate sensors, as the method then
only respectively needs one single integration per angle direction,
as a result of which the precision of the method is improved.
[0072] Even other embodiments than those shown above can be
equivalent and depending on the type of the technical design
(integration algorithms, etc. used) can be of advantage. As an
example of this, reference is made to FIG. 3. There, even the
integration and differentiation could take place at a different
position; the complete integrator block 1b could be eliminated if
simultaneously, the differentiator block 8 would be relocated to
the negative input to the corresponding summation point (see FIG.
7). Only one of the integrators could be eliminated, and only one
of the differentiators of block 8 could be displaced
correspondingly (see FIG. 8).
[0073] Suspension of Calibration
[0074] Under certain circumstances, the calibration process is
suspended for a certain sensor group. Thus, for example, outside of
the water (recognizable at the water contact switch, as already
mentioned previously, or also in the presence of depth information
from the pressure signal of approximately zero), no suitable depth
information is available from the pressure sensor. For this, the
calibration of the straight-line acceleration sensors is suspended,
for example, by setting all confidence factors to zero. The
calibration of the angle sensors can, however, continue to be
operated in it. These, in turn should be suspended when obviously
implausible results are present from the analysis of the magnetic
field sensors (i.e. for example, strong changes of the measured
direction of the magnetic field at only small values of the angle
velocity that are determined with the help of the angular velocity
sensors.
Other Preferred Embodiments
[0075] The path traveled under water can be recorded with the help
of position information from acceleration sensors, angle sensors,
in particular angular acceleration sensors, line-of-sight rate
sensors, magnetic field sensors and/or pressure sensors for storing
the position information depending on time and/or a meter
reading.
[0076] The reference measurement for the coordinates X and Y in the
global coordinate system as well as a sensor calibration can, as
long as, for example, a GPS signal is available close to the
surface, be performed by the GPS system. When receiving the GPS
signals below the surface, in order to increase the precision, a
correction of the delay time of the GPS data with respect to the
propagation properties of the GPS signals under water can take
place. Because of the relatively high dielectric constant of water
of approximately 80, other propagation velocities of the GPS
signals result under water. In particular, the depth information
from GPS signals must hereby be corrected. In the simplest case,
for this, the depth is corrected by a factor of the quotient
between the propagation velocity of the electromagnetic waves in
the expansion space and that under water.
[0077] Most of today's diving computers have a function whereby
they automatically switch on as soon as they make contact with
water. Preferably, this function can be used in order to, for
example, set a reference point at the entry position in combination
with a GPS signal that can still be received at the surface. A is
further reference point can then be used directly when diving in
(start of the dive).
[0078] In particular, the method in accordance with the invention
is expediently useable in combination with a graphic display, on
which the direction and the distance to the reference points is
displayed. The position of the reference points as well as the
previously dived path can be displayed on a map-like illustration.
Corresponding depth information relative to the reference points or
to the path can also be displayed. The path or the reference points
can thereby also be shown in different colors depending on depth,
so that the display remains easy to read, but still provides
additional navigation information to the diver.
[0079] The diver can also set reference points himself during the
dive, for example, by applying pressure to a button. The diver can
also set reference points or destination points even prior to
diving. Further, he can load path points POIs (points of interest)
in advance from sources such as, for example, the Internet into the
diving computer, and thus follow a predetermined path while diving
in order to, for example, find shipwrecks or the hideouts of
certain marine fauna.
[0080] Likewise, previously available map material can be loaded
into the computer of the diver and can thus make the navigation and
orientation easier. For this purpose, the north alignment of the
system can be fixed at the beginning as per magnetic compass and if
necessary, be corrected by long-term averaging. To determine the
north alignment, the movements of the diver can also be determined
by using (still) available GPS signals.
[0081] The dived path, as well as the reference points that were
set can be read after concluding the dive or shown with, or without
a map.
[0082] By radio transmission, even the position of the diver can
perhaps be forwarded to another diver together with other data.
This is particularly helpful when guiding larger groups. This type
of information can also be sent to the diving boat or to the diving
base. One advantage consists therein, that a dive leader, who bears
responsibility, can review the location of a diver on land in order
to perhaps direct the boat to that location, to initiate rescue
operations or to also transmit new reference points or destination
points (for example a new boat position) to the diver.
[0083] Radio transmission, does not only mean electromagnetic high
frequency communication, rather, it covers any type of wireless
communication such as, for example, ultrasound or light.
[0084] To further increase the precision of the depth measured by
the ambient pressure, it can be corrected by the salt content of
the water, and thus the density of the water. A corresponding
measurement of the salt content can be performed by measuring the
conductivity of the water, for example, by electrodes that are
already present for the activation of the diving computer upon
contact with water. In reverse, in the presence of depth
information from other sources (calibrated acceleration sensors,
GPS signal or similar, a determination of the salt content can take
place by a comparison of this information with that of the ambient
pressure sensor. Likewise, manual input, for example, the degree of
latitude can take place.
[0085] A previously known temperature dependence of the sensors can
be compensated by including the signals of a temperature sensor
that is customarily available in a diving computer. Additional
temperature sensors can also be used, that are respectively housed
in the proximity of the sensors.
[0086] An increase in the precision of the calibration of the
sensor can be achieved when the influence of the wave motion on the
pressure measurement is reduced, for example, by forming the
average value of the depth. For this, by using frequency analysis,
the duration of a wave motion can be captured, and thus a favorable
measure for the time constant, or the time period of the average
value formation can be determined (simple or whole number multiples
of the basic frequency). Even a measure for the waviness can be
determined by analyzing the magnitude of the pressure fluctuations.
This analysis can be done in various ways. In one embodiment--in a
frequency range in which the usual wave frequencies occur in the
beach area--the amplitude of the pressure fluctuations is
determined and from this, converted to the fluctuations of the
column of water above the diver. For this purpose, the same known
formula is used as that for the conversion of the water pressure in
the depth. As conversion factor, 10 m/bar is completely suitable.
In the log book of the diving computer, a different recording of
the waviness can then also occur. This recording can take place as
a single value for a dive or also in a sequence of values that
displays the progression of waviness.
[0087] A further increase in precision is achieved by the
correction of the gravitational constant from the degree of
latitude. The gravitational constant 10 (see FIG. 1) is not exactly
the same everywhere, but depends on diverse conditions, in
particular the degree of latitude. Gravity at the equator is 9.7803
m/s.sup.2, at the North Pole 9.8322 m/s.sup.2, and at the 45.sup.th
degree latitude: 9.80665 m/s.sup.2. If information about the degree
of latitude is already available (GPS measurement or manual input)
this signal can be used directly. Thereby, a linear interpolation
between the previously mentioned values can be performed. Other
interpolation methods or methods with several support points can
also be used.
[0088] If no information about the degree of latitude is available,
an estimate of the degree of latitude can be made based on the
water temperature, as the water temperature is naturally higher in
tropical waters than in European degrees of latitude. Hereby, the
salt content can be used additionally, in order to, in the case of
inference of degrees of latitude based on temperature, and the
usually present difference between sweet water and salt water, can
also be included. Even the geographic elevation that can be
determined from the ambient pressure prior to the dive is an
influencing variable that can be included. The variables that can
be considered in the determination of the degree of latitude are,
however, not limited to these. Additional information such as the
time of the year, stored climate zones, etc. can also be
included.
[0089] Even measurements of magnetic fields can be used for the
determination of the degree of latitude. In the case of a diving
computer, the use of magnetic sensors in three dimensions is is
suggested because of the very different orientations in the global
coordinate system. From the z component of the magnetic field,
inferences can be made with respect to the degree of latitude.
[0090] A further preferred embodiment consists of splitting up the
measuring unit and the display unit. While the display unit is
housed on the arm of the diver or in a console, or is integrated as
head-up display in the mask of the diver, the measurement unit
and/or the recording unit can be placed elsewhere. For example, an
attachment at the buoyancy compensator of the diver is suggested,
at the compressed air cylinder or elsewhere on the body of the
diver. This has the advantage that the motions do not take place so
quickly, the angle precision is improved thereby, and the
acceleration values are also reduced in straight-line
direction.
[0091] A further application results from equipping the underwater
propulsion device (for example underwater scooter) with controls
that are controlled depending on the position information obtained
and the direction determined from such to a predeterminable
destination (reference point) or a predeterminable path.
[0092] Deviating from the application shown in the drawing of the
angular acceleration sensors, the line-of-sight rate sensors can be
utilized, as then respectively only a single integration per angle
direction is required, as a result of which the precision of the
method can be improved.
REFERENCE NUMBER LIST
[0093] 1 S acceleration sensor (3 axes) [0094] 1a .intg.
integrators for determining the path from the acceleration in the z
direction [0095] 1b .intg. integrators for determining the depth
from the acceleration [0096] 2 RS (rotation sensor) angular
acceleration sensor (3 axis) [0097] 2a .intg. integrators for
determining the angle from the angular acceleration [0098] 3 T
transformation matrix [0099] 3a transformation matrix [0100] 4
T.sup.1 inverse transformation matrix [0101] 4a T.sup.1 inverse
transformation matrix [0102] 5 log recording device for the
three-dimensional path [0103] 6 .THETA., .PHI., .PSI. determination
unit for solid angles .THETA., .PHI., .PSI. [0104] 7 depth depth
value, for example from pressure measurement [0105] 8 d/dt
differentiators for determining the acceleration in z direction
[0106] 9 A analysis unit for determining the correction factors
[0107] 9a analysis unit for determining the x correction factor
[0108] 10 9.81 value of the actual gravitational acceleration
[0109] 11 correction correction unit for correcting the errors of
the acceleration sensors [0110] 12 path calculation unit,
correcting [0111] 12a path calculation unit, without correction
[0112] 12b path calculation unit, with simple correction [0113] 13
correction value calculation block [0114] 14 weighting unit [0115]
15a X multiplier 1 [0116] 15b X multiplier 2 [0117] 15c X
multiplier 3 [0118] c.sub.Cx confidence factor x axis [0119]
.DELTA.x'' magnitude of error x axis [0120] 18 .DELTA.x.sub.K
correction value for x axis [0121] .SIGMA. summing unit [0122]
T.sub.s time delay (by sample interval) [0123] c.sub.M average
value factor [0124] c.sub.M c.sub.cx product of confidence factor x
axis and average value factor
* * * * *